The Epidemiological Landscape of Thyroid Cancer and Estimates of Overdiagnosis in China: A Population-Based Study.

Background: In China, thyroid cancer (TC) is the most common cancer in females and the fifth most common cancer in males. TC overdiagnosis leads to overtreatment, lifelong medical care, and side effects. This study systematically quantifies the epidemiological profile of TC incidence in China, as well as estimating TC incidence attributable to overdiagnosis. Methods: Data were derived from the Cancer Incidence in Five Continents databases. Age-standardized incidence rates (ASIRs) per 100,000 person-years were calculated using the world standard population. Joinpoint regression and age-period-cohort models were conducted to examine temporal ASIR trends and period effects, respectively. We estimated TC incidence attributable to overdiagnosis among patients aged 15-84 years by comparing shapes of age-specific curves with those observed before diagnostic practices. Results: In 2013-2017, there were 37,862 and 117,979 new TC cases in males and females in China, respectively, with ASIRs of 6.9/100,000 and 21.1/100,000. Significant upward trends were observed from 1993 to 2017, with average annual percent changes being 14.7% for males and 16.2% for females. Incidence rate ratios of period effects markedly increased from 1.0 to 61.8 for males and from 1.0 to 42.9 for females from 1993 to 2017. Overdiagnosis accounted for 83.5% (31,455/37,685) and 88.7% (104,222/117,509) of cases in males and females in 2013-2017, respectively, yielding ASIRs of 5.9/100,000 and 19.1/100,000. ASIRs attributable to overdiagnosis in urban populations (6.9/100,000 for males and 21.7/100,000 for females) were significantly higher than in rural populations (1.3/100,000 and 6.4/100,000). Among the 25 included provinces, ASIRs attributable to overdiagnosis ranged from 0.3/100,000 in Sichuan to 18.5/100,000 in Shanghai in males and from 0.1/100,000 in Shanxi to 49.4/100,000 in Shanghai in females. The national ASIRs attributable to overdiagnosis increased from 2.6/100,000 in 2008-2012 to 5.9/100,000 in 2013-2017 for males and from 9.2/100,000 to 19.1/100,000 for females. Conclusions: The incidence rates of TC have considerably increased over the past 25 years in China. Rapidly increasing trends and considerable geographic variations in TC incidence attributable to overdiagnosis highlight the need to adjust TC screening strategies and clinical practices, optimize healthcare resource allocation, and monitor the impacts of TC overdiagnosis on population-level health.